Over the years, there has been a large number of publications describing the polymerization of ethylenically unsaturated monomers using an organic peroxide as initiator. For example, U.S. Pat. No. 5,155,192 discloses storageable and/or transportable compositions containing peroxydicarbonate to which an organic peroxide has been added to retard the decomposition of said peroxy-dicarbonate. The compositions of U.S. Pat. No. 5,155,192 are suitable for use in the conventional mass, suspension, or emulsion (co)polymerization of ethylenically unsaturated monomers. In U.S. Pat. No. 5,155,192 no further specifications of the peroxydicarbonates to be used are given, such as their solubility or their half life.
An unwanted side effect frequently observed in conventional polymerization processes is the formation of so-called fish eyes in the (co)polymer. One explanation for fish eyes is that they are caused by small quantities of polymer material having a molecular weight that differs considerably from the average molecular weight of the rest of the polymer material. Due to a difference in melt property between said polymer material and the “average” polymer material, irregularities can occur in the final shaped polymer material. It will be clear that this phenomenon is undesirable, for example, for the transparency and uniformity of the final (co)polymer material, particularly in thin films. Furthermore, the presence of fish eyes may even be detrimental to the strength of this material. The presently known conventional polymerization processes have failed to resolve the fish eye problem of (co)polymers satisfactorily.
Hence, it is an objective of the present invention to provide a new process for preparing (co)polymers overcoming the drawbacks of conventional polymerization processes, which process is pre-eminently suited for preparing a (co)polymer with reduced levels of fish eyes at high initiator loads.
US 2002/0123591 discloses the emulsion or suspension polymerization of vinyl chloride monomers, using an organic peroxide that is dosed at least partly to the polymerization mixture at the reaction temperature and wherein essentially all the peroxide has a half life of from 0.05 to 1.0 hour at the polymerization temperature. It is said that a low fish eye level is acquired using this polymerization process.
We have found that (co)polymers with a further reduced level of fish eyes are obtained in an aqueous dispersion polymerization process wherein one or more organic peroxides to be used as initiator are selected from the group consisting of diacyl peroxides, peroxyesters, specific peroxydicarbonates, and mixtures thereof, in conjunction with an effective amount of one or more suitable controlling agents selected from the group consisting of organic hydroperoxides, ethylenically unsaturated organic compounds that preferably cannot homo-polymerize (i.e. form polymers obtained by self-polymerization), compounds with labile carbon-hydrogen bonds, oximes, and mixtures thereof. More preferably, it is an aqueous suspension or emulsion polymerization process. Most preferably, it is an aqueous suspension polymerization process.
We have found that the level of fish eyes is reduced more than would be expected on the basis of US 2002/0123591 by using the process according to the present invention. This is probably due to a synergetic effect caused by using the organic peroxides, controlling agents, and process conditions according to the current invention.